The present invention generally relates to an automatic stitching apparatus for stitching an embroidery pattern on a fabric supported on a tabouret or embroidery frame and, more particularly, to a frame positioning device in the automatic stitching apparatus for moving the tabouret or embroidery frame relative to a stitching needle in a given coordinate direction.
More specifically, the present invention pertains to the frame positioning device wherein a stepping motor assembly is employed for each of two drive mechanisms for respectively moving the embroidery frame in an X-axis direction and in a Y-axis direction perpendicular to the X-axis direction to bring the frame to a predetermined coordinate position.
Japanese Patent Publication No. 42-22489, published on November 2, 1967 in Japan and first filed in the United States of America under U.S. patent applications Ser. Nos. 293,944 and 374,607 of July 10, 1963 and June 12, 1964, respectively, now U.S. Pat. Nos. 3,146,386 and 3,268,785, respectively, discloses a pulse driven positioning system, particularly applicable to an X-Y plotter, which employs stepping motor assmblies for X-axis and Y-axis drive mechanisms.
Although the stepping motor assembly itself may, so far as it is comprised of a plurality of pulse responsive stepping motors wherein, while respective stators remain aligned in phase with each other, the associated rotors are angularly offset at a predetermined angle relative to each other on a power output shaft, be employed in the present invention, it will not meet the following requirements which must be satisfied for the stepping motor assembly to be employed in the present invention.
(1) A high output torque must be available,
(2) A power output shaft must be rotated stepwisely over a predetermined angle as small as possible about the longitudinal axis of the shaft.
(3) The stepping motor assembly must be capable of responding to a train of pulses having a frequency greater than the rated maximum frequency of each of the stepping motors constituting the stepping motor assembly.
However, in order for the stepping motors to be rotated without the output torque falling below an acceptable value, the rate of pulses to be applied thereto must be relatively low. On the contrary thereto, if the rate of pulses to be applied to the stepping motors is increased and in order for the stepping motors to be capable of accurately responding to the applied pulses with no substantial delay in operation, it is necessary to cause the stepping motors to operate so as to give a relatively low output torque. This irreconcilability in performance characteristic of the stepping motors, when the latter are applied in the frame positioning device of the present invention wherein a relatively great force is required to move the frames simultaneously to respective coordinate positions, often provides such a problem that the frames cannot readily be moved to the coordinate positions at relatively high speeds.
Furthermore, to the knowledge of the inventor, the stepping motor assembly specifically designed for use in the automatic stitching apparatus is believed to be novel.